Hello!
By the definition, the average speed is the total distance traveled divided by the total time spent. In our problem the total distance and the total time consist of the two halves.
Denote the first half of the distance as then the second half is also
The time spent during the first half is
where
is the speed of the first half. The time corresponding to the second half is similarly
Thus the average speed is It doesn't depend on
and numerically is equal to
This is the answer. That said, the expression is called the harmonic mean of numbers
and
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